| Abstract: | Sooty blotch and flyspeck (SBFS) fungi colonise the surface of a range of fruits, especially apple, without penetrating the cuticle. Economic damage results from the exclusion of severely affected fruit batches from being marketed as table apples. A study of SBFS was conducted in 2007–2012 in the two largest German apple production areas, i.e. the Lake Constance and Lower Elbe regions. The absence of this disease complex from orchards under integrated pest management in both regions in all years was explained by the collateral effects of scab and storage-rot sprays with captan and quinone-outside inhibitors (QoI) such as trifloxystrobin. However, SBFS was economically relevant in organically managed orchards, being generally more severe in Southern Germany than in the North. In both regions, Peltaster cerophilus was the most frequently isolated SBFS fungus and was chiefly responsible for crop losses. Cyphellophora sessilis, Microcyclosporella mali and Schizothyrium pomi also contributed to SBFS in some organic orchards, whereas a diversity of additional species was confined to untreated orchards. Evidence was obtained that P. cerophilus overwinters within orchards, fruit mummies being one of presumably several colonised plant organs. Infections of young apple fruits were initiated at any time following the end of flowering, and P. cerophilus was capable of causing several infection cycles per season by means of conidial inoculum. The colonisation of sheets of waxed paper by P. cerophilus indicated that this species does not require fruit leachates for growth. No further expansion of colonies was observed during cold storage; instead, P. cerophilus was gradually displaced by other fungi. Differences in the susceptibility of apple varieties to P. cerophilus were due to fruit ripening, late-maturing cultivars being most heavily colonised, and to surface properties, varieties with a waxy bloom being conspicuously less strongly colonised than others. This fungus was unable to colonise russeted fruit areas. Repeated spray treatments with lime-sulphur and potassium bicarbonate throughout the season were effective and necessary to control SBFS in organic production. This strategy threatens the fungicide-saving potential offered by scab-resistant apple varieties. Cultural measures against SBFS include summer pruning as well as the manual removal of fruit mummies in winter. |
